1. Technical Field
The present invention relates generally to an apparatus for rotatably controlling the steering angle of the the rear wheels of a vehicle. More particularly, the invention relates to an apparatus for controlling the steering angle of the rear wheels of a vehicle without increasing the magnitude of overhang of the rear end thereof to improve turning properties so as to achieve the minimum possible turning circle at low vehicle speeds.
2 Background Art
It is well known in the art that extension of the wheel base of an automotive vehicle provides advantages such as improvement of steering stability, drive comfort, and increase space vehicle cabin.
Lengthwise extension of the wheel base however causes the turning properties to be impaired and thus requires a driver to make frequent 3 or 5 point turns instead of cornering in a single motion or making a simple U-turn. Additionally, there is a problem that the vehicle may come in contact with a wall for example due to the difference between turning circles of the inner front wheel and the inner rear wheel. Therefore the road width required for U-turns is increased.
For these reasons, it is difficult to design a vehicle having a relatively long wheel base in view of handling and cornering requirements. For example, in Japan, a car with 3000(cc) displacement has a wheel base of about 2.7(m) as against a car with 1000(cc) having a wheel base of about 2.3(m). It will be noted that the lengths of the wheel bases of both cars are not very different from each other compared with differences in displacement, type, and overall length.
Further, a so-called four wheel steering (4WS) vehicle is well known in the art wherein the provision of rear wheel steering in an opposite phase, or opposite angular direction relative to the steered angle of the front wheels improves the turning properties with respect to the possible minimum turning circle by reducing the turning circle and the difference between the turning circles of inner front and rear wheels. Such 4WS vehicles are already on the market
However, in a conventional 4WS vehicle having a system for steering rear wheels in an opposite angular direction relative to the front wheel steering angle, the rear wheel steering angle is limited to a maximum of 5 degrees and the width between the rear wheels is substantially smaller than the overall vehicle width merely to restrict the magnitude of overhang of the rear end to be less than that of projections such as door mirrors. As the wheel base of a vehicle is increased the more pronounced the design limitations of the steering system become. For example, the turning radius of the front and rear wheels become increasingly different meaning body overhang with respect to wheel position must be increased making it difficult for a driver to accurately estimate vehicle body position during cornering, a situation which could lead to accidents. Therefore, the essential problem in previously proposed 4WS systems is that as steerability (i.e. turning circle) is improved, overhang of the rear end to compensate for the opposite phase rear wheel steering must be increased. This creates problems in cornering, handling and overall vehicle stability.
Therefore, in addition to limitations in modeling with respect to vehicle body, a vehicle having a relatively long wheel base cannot be provided with improved turning properties produced by four wheel steering for the reasons cited above.